The invention is generally directed to a control system for a tissue culture reactor and in particular to a control system for a tissue culture fermentor which exercises concurrent control of dissolved oxygen content (DO) and pH of a medium in a fermenting vessel containing a tissue culture fermentation. Tissue culture fermenting vessels are used to grow cells attached to microcarriers or free suspension cell cultures in a liquid medium containing the various components needed for cell growth. To create the proper environment for the tissue culture cells to grow rapidly and efficiently, relatively narrow ranges of values for concentrations for certain materials are required. These variables include the acidity of the surrounding medium (pH), amount of dissolved oxygen (DO), as well as the concentration of other materials.
These variables include the acidity of the surrounding medium (pH), amount of dissolved oxygen (DO), as well as the concentration of other materials.
The liquid medium for culture growth is agitated by a stirrer, such as a magnetic stirrer, to maintain an even distribution of materials in the surrounding liquid and to attempt to prevent local accumulations of cells and the presence of concentration gradients which can produce undesirable effects upon the tissue culture cells. Cells from higher organisms growing in microcarrier cultures and free suspension cultures are relatively physically fragile and thus only low speed stirring is possible. As a result, a significant potential of harmful concentration gradients is possible where liquid acids or bases are introduced to the fermenting vessel. This makes control of pH by other than addition of liquids extremely desirable, where possible.
Generally, as a tissue culture fermenting process operates, the oxygen and pH needs of the tissue culture change. As a result, there is a need to adjust the flow of materials into the fermenting vessel at various stages of the fermenting process to maintain the tissue culture cells in an optimal growth environment. In addition to maintaining the environmental conditions within ranges of acceptable values, a consistency of values is desired.
It has been determined that the tissue culture cells are responsive not only to the environmental conditions present in the surrounding liquid, but to changes in the environmental conditions of the liquid medium. Therefore, the tissue culture cells often grow more efficiently in a less stressful environment where rapid changes in the surrounding environment are avoided. Therefore, rapid and repeated changes in the surrounding environment are to be avoided.
Two important environmental conditions in a tissue culture fermentor are the amount of dissolved oxygen present in the liquid medium and the pH in the liquid medium. Traditionally, tissue culture fermentor control systems have independently monitored and controlled the level of dissolved oxygen (DO) and pH in the fermenting vessel. However, the independent control of these variables tends to provide for erratic and stressful changes in the environment surrounding the tissue culture.
In particular, when there is an insufficient amount of dissolved oxygen present in the liquid medium, additional oxygen is added to the medium. This has the effect of stripping CO.sub.2 from the medium which raises the pH of the medium (the liquid medium becomes more basic). As a result, the pH level has now shifted away from the desired value and additional CO.sub.2 is added to lower the pH (increase the acidity). However, when CO.sub.2 is added to the liquid medium, it has the effect of stripping dissolved oxygen out of the medium which again serves to require the addition of oxygen to the liquid medium. This cycle operates out of control and the amount of dissolved oxygen in the liquid and the pH of the liquid tend to follow each other causing a stressful environment for the tissue culture.
Accordingly, there is a need for a control system to provide simultaneous regulation of the amount of dissolved oxygen and the pH of the liquid surrounding the tissue culture in a manner which prevents fluctuations in the concentrations of the dissolved oxygen and the pH which provide a stressful environment for the tissue culture.